Disclosed herein are processes for preparing toner compositions. More specifically, disclosed herein are processes for preparing toners with carbon black having low atomic percent values of sulfur.
The formation and development of images on the surface of photoconductive materials by electrostatic means is well known. The basic electrophotographic imaging process, as taught by C. F. Carlson in U.S. Pat. No. 2,297,691, entails placing a uniform electrostatic charge on a photoconductive insulating layer known as a photoconductor or photoreceptor, exposing the photoreceptor to a light and shadow image to dissipate the charge on the areas of the photoreceptor exposed to the light, and developing the resulting electrostatic latent image by depositing on the image a finely divided electroscopic material known as toner. Toner typically comprises a resin and a colorant. The toner will normally be attracted to those areas of the photoreceptor which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. This developed image may then be transferred to a substrate such as paper. The transferred image may subsequently be permanently affixed to the substrate by heat, pressure, a combination of heat and pressure, or other suitable fixing means such as solvent or overcoating treatment.
Numerous processes are within the purview of those skilled in the art for the preparation of toners. Emulsion aggregation (EA) is one such method. Emulsion aggregation toners can be used in forming print and/or xerographic images. Emulsion aggregation techniques can entail the formation of an emulsion latex of the resin particles by heating the resin, using emulsion polymerization, as disclosed in, for example, U.S. Pat. No. 5,853,943, the disclosure of which is totally incorporated herein by reference. Other examples of emulsion/aggregation/coalescing processes for the preparation of toners are illustrated in, for example, U.S. Pat. Nos. 5,278,020, 5,290,654, 5,302,486, 5,308,734, 5,344,738, 5,346,797, 5,348,832, 5,364,729, 5,366,841, 5,370,963, 5,403,693, 5,405,728, 5,418,108, 5,496,676, 5,501,935, 5,527,658, 5,585,215, 5,650,255, 5,650,256, 5,723,253, 5,744,520, 5,747,215, 5,763,133, 5,766,818, 5,804,349, 5,827,633, 5,840,462, 5,853,944, 5,863,698, 5,869,215, 5,902,710; 5,910,387; 5,916,725; 5,919,595; 5,925,488, 5,977,210, 5,994,020, 6,576,389, 6,617,092, 6,627,373, 6,638,677, 6,656,657, 6,656,658, 6,664,017, 6,673,505, 6,730,450, 6,743,559, 6,756,176, 6,780,500, 6,830,860, 7,029,817, 7,459,258, 7,547,499, and U.S. Patent Publication Nos. 2007/0141494, 2008/0107989, 2009/0246680, 2009/0208864, and 2011/0028620, the disclosures of each of which are totally incorporated herein by reference.
Polyester EA ultra low melt (ULM) toners have been prepared utilizing amorphous and crystalline polyester resins as disclosed in, for example, U.S. Pat. No. 7,547,499, the disclosure of which is totally incorporated herein by reference.
Two exemplary emulsion aggregation toners include acrylate based toners, such as those based on styrene acrylate toner particles as illustrated in, for example, U.S. Pat. No. 6,120,967, and polyester toner particles, as disclosed in, for example, U.S. Pat. Nos. 5,916,725 and 7,785,763 and U.S. Patent Publication 2008/0107989, the disclosures of each of which are totally incorporated herein by reference.
Black toners are pigmented polymer composites that employ enough carbon black as the pigment to yield an image with the desired image characteristic after transfer and fusing. The morphology and properties of the carbon black can influence color and electrical charging characteristics. These properties in turn can depend on the uniformity of dispersion of the carbon black in the toner. In emulsion aggregation toners carbon black is dispersed in a liquid phase and then incorporated into the polymer through an aggregation process. There is no mechanical dispersion of the pigment, and yet the carbon black remains dispersed in phases that are chemically different; the amount of shear that can be applied in mixing the toner components is relatively low in an extruder. Accordingly, hydrophilic surface components on carbon black, such as sulfates and the like, inhibit uniform mixing of the carbon black with hydrophobic polymer components.
Carbon black is manufactured via thermal decomposition of hydrocarbons, which are frequently obtained from petroleum feedstocks. Sulfur and sulfur-derived components are common surface contaminants in petroleum-derived carbon blacks. Carbon black comprises spherical particles of elemental carbon fused into aggregates. Manufacturers control the size of the aggregates. Carbon blacks for toner applications balance primary particle size and structure to control color properties, ease of dispersion, and controlled electrical resistivity to allow for the design of charging characteristics. The manufacturers have developed their own proprietary chemical modifications in some cases to alter the surface chemistry of the pigment.
While known compositions and processes are suitable for their intended purposes, a need remains for toners with more reproducible charging characteristics. In addition, a need remains for toners containing carbon blacks containing lower levels of surface contaminants that affect charging characteristics. Further, a need remains for methods of measuring the levels of surface contaminants on carbon blacks used in toners. Additionally, a need remains for toners containing carbon blacks with lower levels of sulfur-containing surface contaminants. There is also a need for toners for which the charge can be stabilized across different temperature and humidity zones.